“Take care that you do not do your righteousness before men,
To be seen of them,
Else you have no reward with your Father who is in heaven.
These words introduce the whole passage from Matthew 6:1-18. The point being made is that in whatever they do, their righteousness (their pious behaviour and good works) is not to be publicly displayed so that men may see it, for otherwise it will result in a total lack of any recompense from their heavenly Father. They will get no spiritual benefit from it. Rather it is to be done in secret in the sight of Heaven, not in the sight of earth.
The idea of ‘recompense' is not that we are to do things in order to get a reward. It is that the reward that the Father gives is so important that it must not be lost by folly, for it involves what we will become and our whole eternal future. It is the reward described in Matthew 5:3-9. It is the consequence of God's active blessing. It is in contrast with receiving the praise of men which will result in a person becoming more proud, more arrogant and more unbearable, and will simply ruin their character. For receiving their Father's reward will make them into precisely what they ought to be for the future.
‘Your righteousness.' The context means that there are two significant meanings to righteousness to be borne in mind here. One is the righteousness of the Scribes and Pharisees in general, which is an insufficient righteousness, and is purely earthly and self-seeking, the other is God's righteousness revealed as active in the believer which will bring a great reward. They have to choose which righteousness they will reveal.
Someone may ask, ‘If our good works are to be seen of men so that they may glorify God as we were told in Matthew 5:16, how can we now be told to keep our works secret?' The answer to this question is simple. It depends on the types of works in mind. In Matthew 5:16 the works shone out because it was unavoidable. They were works done for men's benefit. They thus had to some extent to be known. But they were nevertheless not done to be seen of men but out of obedience to God and in order to bring Him glory. Their being seen of men was simply the inevitable result of that obedience, and was with no desire for men's praise and admiration. Such works could not be kept secret with the best will in the world, but there was certainly no idea that they should be trumpeted abroad.
Here the warning is against behaving in the wrong kind of way in regard to things that can be hidden from men, lest we do them in order to be praised and admired by men. Again the concern is to be that God might be glorified. Thus where possible what they do with regard to these things, almsgiving, praying and fasting, is to be done secretly between them and God. They should not be seeking credit on earth for them. They should be doing them for the glory of God. Even here, however, it may not always be possible to keep the secret. But if the aim has been genuinely to avoid publicity or credit no blame will attach for that. The desire, however, should be that as a result God will again be glorified and not men. However, if men do rather foolishly seek to glorify us then we must immediately turn them away from ourselves towards God, and remind them that they must glorify Him alone.
Further ways in which foolish men sought to do things to earn the praise of men are found in Matthew 23:5-7. This is always a danger when being religious is something that is highly esteemed. But the whole idea of men using religion to bring praise on themselves was seen by Jesus as abhorrent. If such people were genuine their whole concern would be that God be glorified. It would not, of course, be true that all Scribes and Pharisees sought only to glorify themselves. But the problem was that it was true of all too many, and they were the ones who stood out.